The invention relates to a luminescent device containing a luminescent material comprising organic lanthanide complexes comprising organic ligands of which at least one organic ligand is an aromatic ketone.
Such luminescent devices are known from WO-A-98/58037, which describes an electroluminescent device comprising a layer of an electroluminescent material in which the electroluminescent material is an organic lanthanide complex comprising a ligand with the formula: 
Where each of Rxe2x80x3 and Rxe2x80x2 is an aromatic or heterocyclic ring structure which may be substituted or a hydrocarbyl or a fluorocarbon or Rxe2x80x3 is a halogen such as fluorine or hydrogen. When the ligand is to be incorporated into a polystyrene main chain, Rxe2x80x3 has to be functionalized.
It is commonly known that the organic lanthanide complexes may be brought into the excited state via a sensitization process, involving the transfer of electronic energy from the excited triplet state of the organic ligand to the excited state of the lanthanide ion. Consequently, there is a very large Stokes"" shift between the absorption and emission spectra of the complex. A Stokes"" shift as such is advantageous, because it prevents re-absorption of the luminescent light.
However, a very large Stokes"" shift is disadvantageous, because luminescence is only observed when excited with a large excitation energy, and only a portion of the excitation energy is emitted as luminescent light.
For instance, to obtain efficient emission in the blue and green, the organic ligand should have an excited state energy of its triplet state which is at least equal to the energy of the accepting state of the lanthanide ion. This requirement implies that the energy of the excited singlet state is at much higher energy, which typically is in the ultraviolet for the blue and in the violet part of the spectrum for the green emitting species.
It is an object of the invention to provide an organic luminescent device with improved properties with respect to the excitation energy of the organic lanthanide complexes. According to the invention, this object is achieved by a luminescent device according to the first paragraph, which is characterized in that a ringstructure bounded to the ketone moiety of said aromatic ketone comprises a substituted electron donor group. Surprisingly, the absorption bands of said new organic lanthanide complexes shift to longer wavelengths and luminescence is observed when excited with a lower energy.
For instance, when the aromatic Michler""s ketone (4,4xe2x80x2-bis-(N,N-dimethylamino)-benzophenone) co-ordinates to lanthanide xcex2-diketonates, its absorption band shifts to longer wavelengths and in case of Eu(III) complexes sensitized luminescence is observed, which can be excited with wavelengths well beyond 400 nm. Thus far, such long wavelengths for the excitation of sensitized Eu(III) luminescence have not been published, the longest wavelengths of the absorption maxima of luminescent Eu(III) complexes being around 350 nm.
In order to optimize the organic luminescent component with improved properties at the desired color, the nature of the organic ketone ligand according to the invention, the nature of the other organic ligands and the lanthanide ion may be specifically chosen.
It is preferred within the scope of the present invention that said substituted electron donor group is in the para position with respect to the ketone moiety. The para position ensures the most efficient coupling to the ketone moiety which forms a charge transfer complex with the lanthanide ion.
It is also preferred within the scope of the present invention that the other organic ligands of the lanthanide complex comprise xe2x80x94N, xe2x80x94P, xe2x80x94S or xe2x80x94O complexing functionalities. These additional organic ligands are applied in conjunction with an organic ketone ligand to protect the lanthanide ion by shielding it from a direct interaction with quenching bonds, like xe2x80x94OH, xe2x80x94NH or xe2x80x94CH oscillators. Such quenching bonds may induce radiationless deactivation of the excited state of the lanthanide ion, thus leading to a reduced luminescence efficiency.
Said other organic ligands of the lanthanide complex may comprise diketone, triketone moieties.
Alternatively, said other organic ligands of the lanthanide complex may comprise a complexing moiety of general formula: 
Where X is independently CH or N, preferably at least one of the groups X being N, and the bonds a, b, c, and e, and the combination of bonds i/ii and iii/iv are optionally condensed with a benzene group or a condensed aromatic moiety, wherein aromatic carbon atoms may be replaced by nitrogen atoms and wherein the complexing moiety may be substituted with C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, C3-C4 alkylene, CN, halogen, COOH, C1-C3 alkyl-COOH, NO2, NH2, or a pending group for further functionalization or complexation.
It is preferred within the scope of the present invention that the lanthanide ion is selected from Eu(III), Dy(III), Sm(III), Ce(III), Eu(II), Tm(III), Tb(III), Nd(III), Yb(III) and Er(III). In such an embodiment:
Ce(III), Eu(II) and Tm(III) yield blue luminescent light.
Tb(III) yields green luminescent light.
Eu(III), Dy(III) and Sm(III) yield orange/red luminescent light.
Nd(III), Yb(III) and Er(III) yield near infra-red luminescent light
In order to obtain an electroluminescent device, the lanthanide complexes according to the invention are preferably contained in an electrically conducting layer of an organic material. An electroluminescent (EL) device is a device, which, while making use of the phenomenon of electroluminescence, emits light, when the device is suitably connected to a power supply. If the light emission originates in an organic material, said device is referred to as an organic electroluminescent device. An organic EL device can be used, inter alia, as a thin light source having a large luminous surface area, such as a backlight for a liquid crystal display, e.g. for a watch. An organic EL device can also be used as a display if the EL device comprises a number of EL elements, which may or may not be independently addressable.
The use of organic layers as an EL layer in an EL device is known. Known organic layers comprise organic lanthanide complexes as a luminescent compound. The EL device comprises two electrodes, which are in contact with the organic layer. By applying a suitable voltage, the negative electrode, i.e. the cathode, will inject electrons and the positive electrode, i.e. the anode, will inject holes. If the EL device is in the form of a stack of layers, at least one of the electrodes should be transparent to the light to be emitted. A known transparent electrode material for the anode is, for example, indium tin oxide (ITO). Known electrode materials for the cathode are aluminum, magnesium, calcium, lithium, magnesium/silver alloys, and magnesium/indium alloys. The EL device may comprise additional organic layers, which serve to improve the charge transport or the charge injection. Said layers may comprise electron-conducting layers (called xe2x80x98n-type conducting layersxe2x80x99) and hole-conducting layers (called xe2x80x98p-type conducting layers).
A first embodiment of the present invention is characterized in that said lanthanide complexes are contained in an electrically conducting layer of an n-type conducting organic material.
Preferably said n-type conducting organic material is a polymer. This n-type conducting organic layer may be combined with a layer of a p-type conducting material. Said p-type conducting material may be an organic material, which may also be a polymer.
A second embodiment of the present invention is characterized in that said lanthanide complexes are contained in an electrically conducting layer of an p-type conducting organic material.
Preferably said p-type conducting organic material is a polymer. This p-type conducting organic layer may be combined with a layer of a n-type conducting material. Said n-type material may be an organic material, which may also be a polymer.
A third embodiment of the present invention is characterized in that said lanthanide complexes are contained in a layer, which is stacked between a layer of a p-type conducting material and a layer of an n-type conducting material.
Preferably at least one of said p-type or n-type electrically conducting layers is made from an organic material. Said electrically conducting organic material may be a polymer.